In a machine tool having a carriage or slide which moves along a linear guideway, it is sometimes desirable to move the carriage or slide by means of a motor deriven pinion which engages a rack extending along the way. In order to obtain precision movement of the slide without any play, dual drive pinions are used to engage the rack at longitudinally spaced positions, for example, as disclosed in U.S. Pat. Nos. 2,070,807, 2,889,755 and 2,902,875. The dual pinions are driven in such a manner to eliminate backlash between the teeth of the pinions and the teeth of the rack and thereby provide for precision and positive movement of the slide or carriage. As dsclosed in the above-mentioned patents, the reversible drives for the dual pinions and the preloading mechanism are complex and relatively expensive to construct and maintain. it is also desirable to minimize the normal preload torque on the drive pinions in order to minimize wear on the rack and pinions. Thus the normal torque or preload should be sufficient to overcome only frictional resistance of the slide and to maintain light machining of a workpiece. If the preload torque or force must be normally set for heavy or maximum machining forces, the pinions and rack receive excessive wear and require undesirable maintenance.